Electroless plating of copper



MOLS ALKAL/ METAL HYDROX/DE y 1962 F. w. SCHNEBLE, JR., ETAL 3,033,703

ELECTROLESS PLATING OF COPPER Filed Dec. 8, 1958 TOO FAST

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o lllllllllllllllll llllllll MOLS COMPLEX/N6 AGENT INVENTORS ATTORNEYQ 3,033,703 Patented May 8, 1962 3,033,703 ELECTROLESS PLATING OF COPPER Frederick W. Schneble, Jr., Oyster Bay, John MeCormack, Roslyn Heights, and Rudolph J. Zeblisky, Hauppauge, Y., assignors to Photocircuits Corporation, Glen Cove, N.Y., a corporation of New York Filed Dec. 8, 1958, Ser. No. 778,736 9 Claims. (Cl. 117-47) The present invention relates to a process for the electroless plating of copper, and bath solutions used therein.

Early methods for the electroless plating of metals comprised essentially the simple substitution of metals having greater electromotive force by metals having less electromotive force through the medium of an ionizing solution.

It is now conventional to employ solutions containing ions of the metals to be plated in combination with buffers, pH adjusters, reducing agents and the like. A typical sequence of steps using various bath solutions is one in which the surface to be plated is activated or sensitized by one solution seeded by a second solution and finally copper plated from a third solution, as indicated in the simplified flow diagram below:

Wetting Rough with abrasive Rinse Elcctroless copper plating solution Final rinse This type of process is limited in that the solution from which the copper is plated has to be made up in batches which are not renewable. These solutions are further diflicult to use continuously in that once the bath has been started in the process it is progressively used up until it is no longer functional and cannot be interrupted in its course. There is a further difiiculty in that a progressively depleted bath has a constantly changing rate of copper deposition with related changes in the evenness and grain of the resultant plate.

The present invention represents an advance over prior electroless plating baths and the processes in which they are used in that the process of plating may be stopped or started at will and the bath composition is such that it may be used continuously and is not limited to batch usage since it can be replenished. This quality of the bath solutions also allows the composition thereof to be maintained uniform which results in the production of an even, uniformly grained plate.

A further object of the invention is the provision of an electroless plating bath which is economical in use, produces consistently good deposits which are adherent on the surface being plated, and which acts at a predictable rate. Still another object is the provision of a copper plating bath which may be made up over a wide range of copper concentration and within a narrow range ofconcentration of its other ingredients so as to operate at a relatively fast or a relatively slow speed.

The present invention further represents an advance over those processes and baths used heretofore in that the present process can be used to deposit metallic copper without resort to electricity on ordinarily unreceptive surfaces such as plastic surfaces, and on surfaces of metallic copper, iron, nickel, cobalt, aluminum, silver, gold and alloys thereof, such as stainless steel, brass, sterling silver, etc.

It will be understood that the foregoing description and the following detailed description as well are eX- emplary and explanatory of the invention, but are not restrictive thereof.

Surfaces on which copper is to be deposited are generally activated or sensitized by a pretreatment step. The treatment depends upon the chemical nature of the material to be treated and upon its wettability. Readily wetted surfaces require no such pretreatment. The pretreatment of plastic surfaces consists in wetting the surfaces, as by means of a detergent'such as Triton X-l00 a product of the Rohm & Haas Company, of Philadelphia, in a concentration of about 1 ml. of detergent per liter of water. Instead of wetting the surfaces they may be pretreated by roughening said surfaces slightly by the application thereto of abrasive material. The treated surface whether it has been wetted or roughened by abrasion is rinsed thoroughly with water. The rinsed surface is then preferably initially sensitized by treating it with a solution containing grams of stannous chloride (SnCl .2H O) and 40 ml. of hydrochloric acid (37% HCl) per liter of water. The ranges within which these two ingredients may be varied is from 5 to 150 grams of stannous chloride and which may be further acidified by the addition of up to ml. of hydrochloric acid. The surface to be sensitized is usually contacted with this solution for about 10 minutes. In addition to stannous chloride other stannous tin compounds such as stannous fluoborate and stannous sulphate would be suitable in this formulation. The sensitized surfaces are removed from contact with this solution and thoroughly rinsed with water to insure the complete removal of stannous ions from the surfaces. Occasionally it may be desirable to use a rinse Water containing from 0.5 to 25 ml. 'of hydrochloric acid (37% HCl) per liter of water. The removal of soluble stannous ions is necessary since their presence in the second sensitizer or seeder bath would result in its contamination and loss of effectiveness.

The surface to be plated is then preferably further I sensitized by using a second sensitizer or seeder bath which is contacted with the surface which is to receive the copper plating. This bath solution consists of 010 gram per liter of palladium chloride PdCl 1 ml. per liter of hydrochloric acid (37% HQ) and 1 ml. per liter of Triton X-100. The palladium chloride may be substituted for by auric chloride HAuCl in amounts ranging from 0.045 to upwards of 1.8 grams per liter. This preferred composition of the second sensitizer solution or seeder bath may be varied such that the metal content will range from 0.025 gram per liter to 1 gram per liter. The amount of acid may also be increased up to 25 ml. of acid per liter, although the best results are obtained using about 1 ml. of acid per liter. The use of a wetting agent is essential in instances in which the base materials are not easily wetted but are not necessary where the base materials are easily wetted. Contact of the surface with this second sensitizer solution or seeder bath normally lasts for at least two minutes.

The second sensitizer or seeder bath is thoroughly rinsed from the surface which has been in contact with it.

Although in some instances, conventional cooper electroless plating baths may be used after this preliminary treatment if it is not desired to stop and restart the bath, the preferred electroless copper plating bath composition has been found to be the following:

G. Copper sulphate (CuSO -5H O) 14.6 Sodium hydroxide (NaOl-I) 7.5 Rochelle salts (potassium-sodium tartrate) n 7.5 Formaldehyde (37% solution) n 43.8

The amount of sodium hydroxide present in this bath is such that it will provide a pH in excess of 11.0, preferably from 11.6 to 12.5, and optimally 11.75, when diluted with water to make one liter. 1

The foregoing compounds are preferably dissolved in 1 liter of water. The concentration of this solution may be varied, however, by using as little water as 250 mls. and as much as 2000 ads, and the pH will vary accordingly, but always above pH 11.0.

Salts other than copper sulphate may be used which contain either cupn'c or cuprous ions if they are used on the basis of equivalent mols of copper and in a concentration of from 0.02 to 0.25 mol of copper per liter of solution. Among these salts which are suitable for use in place of copper sulphate are cupric and cuprous chloride, cupric carbonate, cupric acetate and cupric and cuprous oxides. Potassium hydroxide may be used in place of sodium hydroxide on a chemically equivalent basis. The tartrate salts are preferably used as complexing agents, because of their economy but sodium citrate or sodium gluconate may be substituted on an equivalent basis with from 0.002 to 0.25 mol of the complexing agent per liter.

The critical relation which must be observed is that the pH must be adjusted so that there is a proper balance between the strength of the alkalinity of the solution and the amount of such complexing agent as may be present.

The diagrammatic figure shows the relation of the molar concentrations of an alkali metal hydroxide, sodium hydroxide as the ordinate, plotted against the molar concentrations of the complexing agent, Rochelle salts 'as the abscissa, up to the maximum concentration of 0.25 mol of Rochelle salts per liter, a value which is not to be exceeded in view of the limit of 0.25 mol of copper per liter of solution.

In such solutions, from 20 to 170 grams per liter of 37% formaldehyde are used, which may be replaced by the equivalent amount of its polymers, paraformaldehyde or trioxane.

It is also important that the concentration of copper salt in the bath be kept below 0.25 mol and that there also be more copper ions in solution than Rochelle salts on an equivalent basis. Under these conditions the bath will have a clouded appearance.

In order for the deposition of copper to proceed at a reasonable speed it is necessary that the pH of the bath be at least 11.0 and that the equivalent concentration of the sodium or potassium hydroxide be at least five times that of the Rochelle salts. It is also important that the mol concentration of the sodium hydroxide or potassium hydroxide should not exceed seven times the mol concentration of the Rochelle sa1ts.

Examples of other copper plating batches which may be used are:

G. Oupric chloride (CuCl 7.8 Sodium hydroxide; 7.5 Rochelle salts 7. 5 Formaldehyde (100% conc.) 16.2

G. Copper carbonate (CuCO 7.5 Potassium hydroxide (KOH) 10.7 Rochelle salts 7. 5 Paraformaldehyde (100%) 16 Water to make a liter of solution.

A fast working concentrated bath according to the present invention:

G. Copper sulfate (CuSO -5-H O) 58.4 Sodium hydroxide 57.0 Rochelle sal 57.0 Formaldehyde (100%) 120.0

Water to make 1000 solution;

A much slower Working concentrated bath would have the following composition:

Copper sulfate (CuSO -5H O) 58.4 Sodium hydroxidencu- 37.5 Rochelle salts- 57.0

Formaldehyde 120.0 Water to make 1000 ml. solution.

A more dilute bath, which is economical to establish has the composition:

Water to make 1000 ml. solution.

It is possible to stop the plating of copper from these baths by the adjustment of their pH. When the pH of the bath is shifted to the acid side by the addition of a suitable acid, the reduction and plating out of copper stops. The adjustment of the pH may be accomplished through the use of an acid, sulphuric being most economical, and may be restarted by raising the pH to above 11.0.

The replenishment of certain compounds in the baths composition over periods of use usually comprise the addition of a suitable copper salt, the addition of a complexing agent, the renewal of the reducing agent and addition of alkali to maintain the desired pH in excess of pH 11.0.

The invention in its broader aspects is not limited to the specific steps, methods and compositions described, but departure may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A bath for the electroless plating of copper which consists essentially of from 0.02 to 0.25 mol of a water soluble copper salt, from 0.25 to 2.1 mols of formaldehyde, from 0.002 to 0.25 mol and in an amount less than that necessary to completely complex said copper salt of a copper complexing salt selected from the group consisting of Rochelle salts, sodium gluconate, and sodium citrate, alkali metal hydroxide from 5 to 7 times the number of mols of the copper complexing salt and sufficient to give the bath a pH of at least 11.0 and water sufficient to give one liter of bath solution.

2. A bath as set forth in claim 1 wherein the complexing agent is Rochelle salts.

3. A bath as set forth in claim 2 wherein the pH is adjusted to within the range 11.6 to 12.5.

4. A bath as set forth in claim 1 where the complexing agent is present in an amount of between 0.01 and 0.25 mole, said amount being less' than that necessary to completely complex said copper salt;

5. A bath for the electroless deposition of copper which consists of:

G. Cupric sulphate 14.6 Sodium hydroxide 7.5 Rochelle salts 7.5 Formaldehyde 16.2

Water, 250 to 2000 ml.

6. A bath as set forth in claim 5 wherein the pH is adjusted to 11.75.

7. A process for the electroless plating of copper which comprises immersing a surface to be plated in an aqueous bath consisting essentially of a water soluble copper salt in an amount from 0.02 to 0.25 mol, from 0.25 to 2.1 mols of formaldehyde, a copper complexing salt selected from the group consisting of Rochelle salts, sodium gluconate and sodium citrate from 0.002 to 0.25 mol and less than that necessary to completely complex said copper salt, an alkali metal hydroxide sufiicient to give the bath a pH of at least 11.0 and in a molar quantity of from 5 to 7 times the number of mols of the copper complexing salt in each liter of solution, maintaining the surface in said bath until a deposit of the tion containing stannous ions, washing said surface, seeding the sensitized surface with a solution containing ions selected from the group consisting of palladium and gold, washing said surface preliminary to contacting the surface thus prepared with an electroless copper plating bath as claimedv in claim 1.

9. A process as set forth in claim 8 in which the copper depositing bath consists of:

. G. Cupric sulfate 14.6 Sodium hydroxide 7.5 Rochelle salts 7.5

Formaldehyde 16.2 Water, 250 to 2000 ml.

References Cited in the file of this patent UNITED STATES PATENTS 2,454,610 Narcus Nov. 23, 1948 2,702,253 Bergstrom Feb. 15, 1955 2,872,359 Saubestre Feb. 3, 1959 2,874,072 Cahill et al. Feb. 17, 1959 2,930,106 Wrotnowski Mar. 29, 1960 OTHER REFERENCES Wein: Metallizing Non-Conductors, Metal Industry Pub. Co. (1947), pp. 40-42. 

1. A BATH FOR THE ELECTROLESS PLATING OF COPPER WHICH CONSISTS ESSENTIALLY OF FROM 0.02 TO 0.25 MOL OF A WATER SOLUBLE COPPER SALT, FROM 0,25 MOL TO 2.1 MOLS OF FORMALDEHYDE, FROM 0.002 TO 0.25 MOL AND IN AN AMOUNT LESS THAN THAT NECESSARY TO COMPLETELY COMPLEX SAID COPPER SALT OF A COPPER COMPLEXING SALT SELECTED FROM THE GROUP CONSISTING OF ROCHELLE SALTS, SODIUM GLUTNOTE, AND SODIUM CITRATE, ALKALI METAL HYDROXIDE FROM 5 TO 7 TIMES THE NUMBER OF MOLS OF THE COPPER COMPLEXING SALT AND SUFFICIENT TO GIVE THE BATH A PH OF AT LEAST 11.0 AND WATER SUFFICIENT TO GIVE ONE LITER OF BATH SOLUTION.
 8. THE PROCESS OF ELECTROLESS PLATING COPPER, THE STEPS OF SENSITIZING THE SURFACE TO BE PLATED WITH A SOLUTION CONTAINING STANNOUS IONS, WASHING SAID SURFACE, SEEDING THE SENSITIZED SURFACE WITH A SOLUTION CONTAINING IONS SELECTED FROM THE GROUP CONSISTING OF PALLADIUM AND GOLD WASHING SAID SURFACE PRELIMINARY TO CONTACTING THE SURFACE THUS PREPARED WITH AND ELECTROLESS COPPER PLATING BATH AS CLAIMED IN CLAIM
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